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Chapter 10States of Matter
10.4 Changes of State
Things you will learn
• You will be able to interpret the phase diagram for water at any temperature and pressure
• You will understand and be able to describe how solids move to the gas phase and recondense as solids without going through a liquid phase
Things we might know about the three phases of matter
• Solids, liquids and gases have different amounts of kinetic energy
• Solids, liquids and gases have different amounts of order in their structures
• The forces which hold together each phase of matter are different– Gas – no attractive forces (kinetic theory) – Liquid – weak intermolecular forces– Solid – strong ionic or weak intermolecular forces
Phase diagrams
• The relationships between solid liquid and vapor of a substance in a sealed container are sometimes displayed with a phase diagram.
• We saw that the boiling point of water was affected when we put a bottle of water in the vacuum chamber. Is it possible that all kinds of unexpected things happen when we change the pressure on a system containing a substance?
Sublimation
• Icy sidewalks become dry even when the temperature does not go above freezing
• The ice goes from a solid directly to a vapor phase• This is because solids, like liquids, have vapor pressure• If this vapor pressure is high enough, the substance will
pass from a solid to a gas without first becoming a liquid• Look at all the old round and tiny ice cubes in your
freezer left over from a party a month ago• Look at the glass in your car on a hot day (maybe not
quite the same thing)
CO2 sublimation
CO2 phase diagram
Vapor pressure of CO2
Slow motion CO2 vapor build-up
“fun” with dry ice
Pressure Explosion. Carbon dioxide has a vapor pressure of 830 psi at 20 °C. In other words, if one places solid carbon dioxide in a closed container at room temperature, the carbon dioxide will eventually convert to liquid and the pressure above that liquid will be 830 psi (approximately 56 times normal atmospheric pressure). The pressure will always be 830 psi as long as some liquid carbon dioxide is present in the closed container, and at higher temperatures the pressure will be even greater. While such pressures are no problem for pressure-tested gas cylinders or fire extinguishers, ordinary containers (soda bottles, paint thinner cans, Thermos containers etc.) can not handle such pressures and will explode and create shrapnel if solid carbon dioxide is sealed inside them. Putting solid carbon dioxide into any sort of closed container (glass, plastic, metal etc.) is exceedingly dangerous and is likely to result in severe personal injury or death. If, after reading the above paragraphs, you are even thinking about making a carbon dioxide bomb as a "prank", you are a complete idiot. We have personally read the full medical report of a middle school student who lost an eye to one of these. Not only is the risk of injury or death very high, in many states constructing, possessing or using such a device is a felony. See Salt Lake Police Department's Bomb Squad warns against homemade chemical bombs.
